1. Field of the Invention
The invention relates to a process for protecting from oxidation and firing a non-ferrous liquid metal, in particular liquid magnesium, in a machine processing this metal or its alloys.
2. Description of the Related Art
It is known, in casting machines for manufacturing ingots of magnesium or its alloys, to protect the magnesium from the phenomena of oxidation and firing by injection at its surface inerting or inhibiting gaseous mixtures constituted by one or more gases such as carbon dioxide, air, nitrogen and argon, containing gases such as sulphur hexafluoride (SF6), or other fluorine-containing compounds such as BF3, (CCIF2)2, or sulphur dioxide (SO2). However, given the nature of the gases injected in this way, this manner of proceeding involves major risks to human health and to environment.
The object of the present invention is to overcome the aforementioned drawbacks and, for this purpose, it provides a process for protecting a non-ferrous liquid metal from oxidation and from firing in a machine processing this metal or its alloys, consisting in disposing a gaseous mixture containing carbon dioxide at the surface of the non-ferrous metal, and involving practically no risk to the health of the operators or to the environment, since it uses neither the aforementioned fluorine-containing compounds nor SO2.
According to the present invention, the process is characterised in that the gaseous mixture is prepared by adding to the carbon dioxide an oxidising gas, argon and xenon only, this gaseous mixture having properties ensuring protection from the ambient atmosphere, and the proportion of carbon dioxide in this gaseous mixture ranging from 50% to 90% by volume.
Advantageously, the proportion by volume in the gaseous mixture of the oxidising gas ranges from 5% to 15%, that of the argon is at most 40% and that of the xenon is at most 10%
Advantageously, according to the invention, the components of the gaseous mixture are dried before they come into contact with the metal.
The respective roles of each of the four constituents of the gaseous mixture according to the invention are as follows:
carbon dioxide (CO2) is a heavy gas, and further has properties of adsorption on magnesium oxide (MgO), which reinforces the barrier effect of the latter;
the oxidising gas makes it possible to obtain at the surface a layer of magnesium oxide that acts as a barrier;
argon, which is a heavy, non-oxidising gas, and which is insensitive to magnesium radiation at high temperatures, ensures that the temperature rise of the gaseous mixture, and thus the convection phenomena associated therewith, are limited;
xenon, which is a very heavy, inert gas, and which is also insensitive to magnesium radiation at high temperatures, gives the mixture a greater specific gravity.
It is the combination of the respective properties of these four components, and the interaction between them, that provides optimal protection for liquid metal, in particular for magnesium.
To ensure that the invention is clearly understood, a description will be given below, by way of a non-limitative example, of a preferred form of embodiment, with reference to the annexed drawing, the sole FIGURE of which is a partial schematic view of a magnesium ingot casting machine.
With reference to the drawing, there is shown a casting machine including a casting line 1 carrying a series of ingot moulds 2 continuously travelling past a casting tube 3 via which magnesium in a liquid state is deposited in ingot moulds 2.